The present invention relates to a surface-emitting semiconductor laser and a method for manufacturing thereof as well as an optical device including a light-source device in which the surface-emitting semiconductor laser is used. In particular, the present invention relates to a surface-emitting semiconductor laser in which expansion of the mode-field diameter allows for an increase in output with maintaining a single horizontal mode and a method for manufacturing thereof as well as an optical device.
A surface-emitting semiconductor laser is used, for example, as a light-source device for the optical communication when the data communication is performed.
A high power output in such a surface-emitting semiconductor laser is conventionally attained by making an emitting area large. However, when an emitting area is made large, conventionally a single horizontal mode becomes a multi-mode.
On the other hand, an optical fiber having a single horizontal mode is used in the long distance transmission, for example, and favorably, input laser light for that transmission is a single horizontal mode to obtain a high coupling efficiency and a high transmission efficiency. A high power output laser of the single horizontal mode is required for the above reason and the like.
In addition, high speed operation is recently required with respect to an optical pickup device used for recording and reproducing on an optical recording medium, and regarding the laser light used as the light-source the increase of the output becomes indispensable while maintaining the singularity of a horizontal mode. Also, the single horizontal mode is indispensable in a printing device such as a laser printer and others, and also the high power output is indispensable in the high-speed processing, the laser in which the single horizontal mode and the high power output are both obtained is required for those optical devices.
A surface-emitting semiconductor laser using a photonic crystal structure was proposed as the surface-emitting semiconductor laser having the above described single horizontal mode. See generally, Japanese Patent Application No. 2002-292781; and Noriyuki Yokouchi, et al, Appl. Phys. Lett. 82, 3608(2003).
Recently, using the photonic crystal structure, the horizontal mode of the surface-emitting laser is controlled to obtain both the expansion of the mode-field diameter that leads to the increase of output and the single horizontal mode.
However, control of the horizontal mode of the surface-emitting semiconductor laser by the photonic crystal is performed based on the concept of decreasing an effective refractive-index difference between the resonator portion that confines the mode and surroundings thereof so that a high-order-mode is made to be the leakage mode. Therefore, the effective refractive-index difference must be decreased in size because as the resonator portion diameter for the increase of output expands in size, it becomes increasingly difficult for the high-order-mode to leak. Accordingly, the principle of a single horizontal mode is extremely weak against the outer disturbance which makes refractive-index distribution occur.
Also, in the surface-emitting semiconductor laser which has a large diameter, an area on the side surface of the resonator portion that functions for the leakage loss decreases in the volume ratio of the resonator portion, so that the Q value of all the modes which include the leakage mode increases. On the other hand, as for a basic mode originally having a small leakage loss and having the large basic value, since a parasitic absorption and scattering-loss remains in the laser, an increase of the Q value is restricted, and an extremely high Q value can not be obtained. Accordingly, since the large Q value ratio between the modes cannot be maintained, the function of selecting a mode is deteriorated.
Even in the laser in which the Q value ratio between the modes is small and the mode selecting function is weak for the outer disturbance as described above, the mode field diameter larger than a conventional single horizontal mode type surface-emitting semiconductor laser is certainly obtained in the case where the drive current is small and the ideal effective refractive-index distribution and carrier density distribution are realized. However, with the increase in the drive current, since the refractive-index distribution which occurs with the rise of temperature when applying electricity and the space gain hole-burning phenomenon in which the mode localized region is selectively come short of a carrier may occur, the condition to maintain the single horizontal mode deteriorates easily, and there is a problem that the horizontal mode becomes a multi mode after all.
Accordingly, a structure to maintain a more stable the single horizontal mode has been required.